The present invention relates to inorganic colorants, pastes containing such colorants dispersed therein, and paints containing the colorants and/or the pastes. In more detail, this invention relates to inorganic colorants which are useful for producing coloring coatings etc.; pastes, in which the colorants are dispersed beforehand, which are used in the production of paints etc.; and paints produced from the inorganic colorants and the pastes in which the colorants are dispersed.
As paints, there are so-called oil-based paints, which contain one or more organic solvents; water-based paints, wherein water is used instead of the organic solvent; and powder paints, in which a solvent is not used. Since the oil-based paints involve the use of an organic solvent which is harmful and volatile, there is a potential problem of pollution occurrence, such as air pollution, and there is the possibility of injury to the health of workers where the coating is being made or where it is being applied. Therefore, water-based paints and powder paints are in developing use.
In formulating a paint, usually one or more powdered colorant is ground in a vehicle of a dispersion medium, which comprises suitable resins, solvents and so on, to produce a so-called paste in which colorants are substantially uniformly dispersed, and then resins, solvents, and various kinds of additives are further added to the paste, if necessary.
Both organic and inorganic colorants have been used as the raw materials for paints. While the organic colorants are superior in color clarity, the inorganic colorants show high stability and are superior in heat-resistance, hiding power, and weather-resistance. Therefore, inorganic colorants are more often chosen for those uses in which durability is necessary.
From the standpoint of efficiency of production, a paste which contains a colorant in high concentration is at first prepared by using various types of dispersing means, and this paste is then diluted with a solution of a resin, which is selected depending upon the potential use, to get a paint. In this process, it is very important to disperse the colorant particles in a fine, stable condition and, for this, the colorant particles must have a good affinity for the resin molecules which comprise the paste. It is also important for the adsorbed resin to prevent flocculation of the dispersed colorant particles (that is to stabilize the colorantdispersion) by formation of an adsorbed resin layer on the colorant particle surface. Furthermore, paints and paint films have to possess and achieve certain performance criteria, such as gloss, hiding power, stability against settling, flooding, floating, and so on. In order to enhance these kinds of performance, fine dispersion of the colorant is necessary. Dispersibility of the colorant in the paint is improved as the affinity between the colorant and the vehicle increases, and the affinity between the colorant and vehicle is affected by the surface properties of the colorant.
The vehicle in which the colorant is dispersed is usually composed of resins as aforementioned. In the case where the resin molecule has an acidic functional group, such as a carboxyl group or a sulfonic acid group; or a basic functional group, such as a primary, secondary, or tertiary amino group, or an imino group, the resin molecule adsorbs the colorant by an acid-base interaction. Dispersion stability is therefore achieved when a basic functional group or an acidic functional group, respectively, is suitably introduced into the colorant.
In this connection, among the known methods which improve the affinity between the colorants and the resins, is the art wherein amphoteric resins are used to enhance the acid-base interaction between resins and colorants, see published U.S. Pat. No. 4,632,961 (Japanese Official Patent Provisional Publication, Showa 58-21,468), and the technology wherein acidity and/or basicity is imparted to colorants by plasma-treating to enhance the acid-base interaction between resins and colorants, see published U.S. Pat. No. 4,478,643 (Japanese Official Patent Provisional Publication, Showa 58-217,559).
In the series of water-soluble resin paints, the acidic or basic functional group has been consumed (neutralized) when the resin was dissolved into a solvent (water). It is generally difficult to utilize the acid-base interaction for the adsorption of the resin on the colorant.
In this series of paints, adsorption of the resin on the colorant is attained by hydrophobic interaction so that the free energy at the interface between the colorant and the resin is smaller than the free energy at the interface between the colorant and water. Since the inherent molecular skeleton of water soluble resins have low polarity, and organic colorants and carbon black have low polarity on their surfaces, the free energy at the interface between the colorant and the resin is small, and absorption of the colorant by the resin proceeds easily.
However, most inorganic colorants are composed of metal oxides, such as TiO.sub.2, ZnO, SiO.sub.2, Al.sup.2 O.sub.3, Fe.sub.2 O, FeO, and Fe.sub.2 O.sub.3, or of metals, such as Al, Cu, and Zn. Many hydroxy groups are formed on the surface of these colorants by the chemical or physical adsorption of water, which exists as humidity in the air. Because of this, the surface of inorganic colorants generally have high polarity and, for example when the polarity is indicated by the heat of immersion in water, the organic colorants, such as phthalocyanine blue and quinacridone red, and carbon black, show a polarity of about 0.1 J/m.sup.2 or less, whereas the polarity of the inorganic colorants, measured in the same way, are about 0.25 J/m.sup.2 or more. Thus, the inorganic colorant surface has a high polarity, so that the free energy at the interface between the colorant and the resin is large. A relatively unstable interface results, and resin adsorption does not readily proceed. Therefore, in order to increase the affinity between inorganic colorants, having such an inherently polar surface as above, and resins which have relatively lower polarity, the polarity of the surface of the inorganic colorant must be lowered. That is, in the series of water-soluble resin paints, since dispersion stability is mainly attained with the resin adsorption resulting from the hydrophobic interaction between the resin and the colorant (T. Kobayashi, T. Terada, and S. Ikeda, J. Japan Society of Colour Material, 62, 524 (1989)), the colorant surface has to be hydrophobic (low polarity) and thus, it is necessary to decrease polarity of the inorganic colorant surface.
Regarding the modification of the colorant surface, the following methods are exemplary of methods which have been recently been proposed:
as a wet method:
(1) treating the colorant surface with a silane coupling agent, or PA1 (2) treating the colorant surface with an aliphatic alcohol and a fatty acid, etc. at elevated temperature: and PA1 (3) vapor phase fluorination using a high temperature flow reactor (published in the Bulletin of the Chemical Society of Japan, 60, 2833 (1987)), or PA1 (4) polymerization of a cyclic silicone compound on the surface of a colorant (published in the proceedings of the Annual Meeting of the Japan Society of Colour Material, 1986, 1B-10).
as a dry method:
In the said hitherto known methods for treating the surface of colorants, there has been some difficulty in selection of the treating conditions. Further, the cost of treatment is high. In method (2), the treatment is sometimes not possible the chemical properties of the surfaces of colorants. For example, surface treatment with an aliphatic alcohol is limited to those situations where the surfaces of the colorants are neutral or weakly acidic. In addition, if such treated colorants are used under aqueous conditions, the treated layer may be removed from the surface of the colorants by hydrolysis and, therefore, these colorants cannot be used in a water-based paint.
Method (3) needs to use such a high temperature, about 700.degree.-800.degree. C., for its reaction, that the hue sometimes changes due to exposure to such high temperature. As for method (4), applicable colorants and vehicles cannot be freely chosen because a special silicone compound is used.